Trichlorosilane (SiHCl3) is a compound useful as a starting material for the production of polysilicon. Upon reacting trichlorosilane with hydrogen at a temperature as high as 1000° C. or more, polysilicon precipitates in a highly pure form. This reaction is expressed, chiefly, by the following formulas (1) and (2),4SiHCl3→Si+3SiCl4+2H2  (1)SiHCl3+H2→Si+3HCl  (2)
Trichlorosilane is, usually, produced by reacting metallic silicon with hydrogen chloride. For example, a patent document 1 discloses a process for producing trichlorosilane by reacting metallic silicon with hydrogen chloride in the presence of a catalyst containing iron and aluminum by using a fluidized bed-reaction apparatus.
According to the above production process, trichlorosilane is formed from metallic silicon and hydrogen chloride by the reaction of the following formula (3),Si+3HCl→SiHCl3+H2  (3)
On the other hand, tetrachlorosilane (SiCl4) by-produced at the time of producing polysilicon can be converted into trichlorosilane and can be reused for the production of polysilicon. For instance, a patent document 2 discloses a process for producing trichlorosilane by reacting metallic silicon, tetrachlorosilane and hydrogen in a fluidized bed in the presence of a copper silicide catalyst. According to this production process, trichlorosilane is formed from metallic silicon, tetrachlorosilane and hydrogen according to the following reaction formula (4),3SiCl4+2H2+Si→4SiHCl3  (4)
This production process, however, has such defects as requiring a reaction temperature higher than that of the trichlorosilane producing process according to the above formula (3), low reaction rate and high cost of production. Therefore, trichlorosilane has been produced chiefly by the reaction of metallic silicon with hydrogen chloride (reaction of the formula (3)), while the production process of the formula (4) is carried out additionally for reusing tetrachlorosilane that is by-produced in the production of polysilicon.
Here, the process for producing trichlorosilane by the reaction of metallic silicon with hydrogen chloride (reaction of the formula (3)) is accompanied by the side reaction of the following formula (5) forming, as by-products, tetrachlorosilane and dichlorosilane in trace amounts,Si+4HCl→SiCl4+2H2  (5)
Therefore, a gas formed by the reaction of metallic silicon with hydrogen chloride is cooled down to −10° C. or lower to separate trichlorosilane by condensation. Here, the condensed solution also contains other chlorosilane compounds which are by-produced in addition to trichlorosilane. This trichlorosilane is separated and recovered by distillation from the condensed solution that contains these chlorosilane compounds, and trichlorosilane which is recovered is used as a starting material for producing polysilicon. Further, tetrachlorosilane that is separated by distillation is chiefly reused for the process for producing trichlorosilane that is additionally conducted relying upon the reaction of the formula (4).
The waste gas after chlorosilanes are separated by condensation from gases formed by the reaction comprises chiefly hydrogen but still contains unreacted hydrogen chloride, chlorosilane compounds that are remaining in small amounts without being separated by condensation as well as trace amounts of boron and the like that are unavoidably contained as impurities in metallic silicon. The impurities lower the quality of polysilicon. Therefore, trichlorosilane used as the starting material for producing polysilicon must not contain impurities as much as possible. Because of this reason, the waste gas containing impurities was partly recirculated as a carrier gas into the reaction system of metallic silicon and hydrogen chloride but was mostly disposed of through a suitable treatment.
However, the amount of the waste gas to be disposed of after chlorosilane compounds are separated is on the increase accompanying an increase in the amount of production of trichlorosilane, and it has been urged to establish a method of effectively reusing the waste gas.
On the other hand, a patent document 3 is proposing a process for producing trichlorosilane by feeding granular metallic silicon, hydrogen chloride, tetrachlorosilane (silicon tetrachloride) and hydrogen into a fluidized bed reactor filled with granular metallic silicon so that there simultaneously proceed a trichlorosilane-forming reaction of metallic silicon with hydrogen chloride (reaction of the formula (3)) and a trichlorosilane-forming reaction due to the reaction of metallic silicon, tetrachlorosilane and hydrogen (reaction of the formula (4)).